VRLA Batteries: usage in solar systems

Valve regulated lead acid (VRLA) batteries

A VRLA (Valve-Regulated lead acid) batteries are commonly known as sealed or maintenance free batteries. As the name ‘valve regulated’ defines, these batteries have a safety valve, which allows the release of gases produced.

These batteries use recombinant technology for operations which means oxygen gas is produced on the positive plate and is absorbed by the negative plate. The negative plate produces hydrogen which combines with oxygen to produce water – this way the water content in the battery is maintained.

VRLA batteries are of two types –

Gel batteries

AGM Batteries

Advantages of using VRLA batteries for your solar system

VRLA has numerous advantages over traditional flooded batteries.

A flooded or wet cell battery utilises electrolyte as sulphuric acid which is a dangerous acid if spilled or externally comes in contact to anything. VRLA batteries don’t contain any such liquid, as their electrolyte is formed as gel and absorbed by separator sheets.

VRLA batteries are spill proof and can be installed in almost any location.

Flooded batteries require proper ventilation set up for gas escaping, while valve regulated batteries allow for installation in places that have limited ventilation.

Common applications of VRLA batteries

VRLA batteries are used in many different industries, not just in solar:

Deep Cycle, Deep Discharge Applications

Marine Trolling and Electronics

Sailboats and Electric Vehicles

Wheelchairs / Scooters and Golf Cars

Portable Power and Floor Scrubbers

Personnel Carriers and Renewable Energy

Village Power (Solar, Wind)

Marine & RV

House Power Cycle Applications

Standby and Emergency Backup Applications

UPS (Uninterrupted Power Supplies)

Emergency Lighting

Computer Backup

Frequency Regulation (Solar, Wind)

Telephone Switching

Other Applications

Race -Highway Trucking

Performance Cars and Off-Road Vehicles

Diesel Starting and Light Trucks

Start-Stop Systems

Charging of VRLA batteries

For charging of VRLA batteries two methods can be used –

Constant Current which means that the output voltage of battery charge controller is varied keeping the current constant. This type of charging is suitable for batteries being used in cyclic applications as a traction battery and it is required that the charger is removed from the battery when the battery is fully charged.

Not suitable for stand by and emergency applications such as UPS, laptop and computer back-ups.

Constant Voltage, which means same voltage is applied by varying the current throughout the charging of battery. Due to the potential difference between the charger and discharged battery, the initial currents are high, voltage varies and SOC increases. The initial charging of battery is quick but takes some time to get fully charged.

Overcharging or undercharging

As it is known that VRLA works according to the recombinant principle, charging above the limit causes excess oxygen and hydrogen production, which escapes through the safety or pressure relieve valve. This may lead to a dry condition in the battery. Since water cannot be added to the battery, this could lead to capacity loss or even permanent damage.

If the battery is undercharged, the cell leads to sulfation. Sulfation is a phenomenon in which the sulphate from the cell goes back to the electrolyte during recharging of cells. Undercharging causes the sulphate retained on the cell plates causing reduction in capacity. In the long run this might cause cell failure.

Keeping these two key points in mind and a little care will help the durability and longevity of VRLA Batteries.

By chris on 14 October 2015

it really depends on many factors, e.g. your loads, power consumption behaviour (relatively even/ peak), days of autonomy, also on the location where you are based as it comes with impacting local weather data.

As an example: let’s say you go off-grid with a PV system to meet your daily electricity needs and you want to use batteries as backup during the time the modules don’t provide enough power (e.g. during night), no autonomies of several days, just simple charge and discharge during night; to start with you need to know the maximum possible total watt hours the batteries need to provide for as well as the technical specs of your battery in order to calculate how many of those you will need.

Continuing the example, your light bulbs, TV, notebook, fridge, kettle and other loads are 2400 watts and would need to be supported by a battery during maximum 5 hours any given day in a year, so that is in total 12000Wh. Say our battery is a 12V/200Ah VLRA, with 2400W of loads; that would be 2400W/ 12V = 200amps, and with a daily potential load-powering maximum of 5h, with this 12V battery we need 5h x 200amps = 1000Ah

Given that our sample VRLA is 12V/200Ah, we would at least need: 1000Ah/200Ah = 5 VRLAs. Why ‘at least’? As explained in our storage technology/ learning center articles, you should not discharge the battery completely so to prolong its lifetime. Usually up to 1/2 (50%) discharge should be fine, however there are other traps of capacity losses caused by the cables, inverter, self-discharge etc., so you may end up with discharge limit at say 40%, which is about 13 batteries.

Keep also in mind that for efficiency and longevity for all the batteries in the bank, they should be interconnected properly so to avoid unequal treatment = loads draw excessively current from one battery in the bank while the others are less drawn from.

By Pau on 8 November 2017

I am working in a telecommunication industry specializing in telecom power. During my 2 years experience using VRLA (2V, 24 cells, 48V system arrangement as with all the telecom towers) batteries, I have discovered that those use in cyclic operation (meaning charge to 90% and discharge to 50%) last longer than standby operation (meaning almost 24 hour AC supply is available, so the batteries are kept at 100% SOC with nominal float charging voltage 2.27 as per manufacturer’s manual). Both operations involves refreshing charging/ equalization charging which charges the batteries for 24 hours at 100% SOC with 2.4V as per manufacturer’s manual, every 30 days. If we assume that the same load 2kW is used for both operations, both systems having 1000Ah battery capacity, all the batteries are fully charged and AC source disconnected, systems with cyclic operation would give more back-up time than the one with standby operation.
It would be very much appreciated if I can get a simple understanding of what that means.